This invention relates to optical filters and, in particular, to optical filters utilizing birefringent materials.
In a number of applications of optical technology, it is necessary to isolate a particular wavelength or band of wavelengths from incident light containing a broad band of wavelengths. In the field of infrared detection, for example, a typical target object, such as an airborne vehicle, can be identified because it will emit a characteristic infrared radiation spectrum containing features which are unique to that type of vehicle, such as one or more peak amplitudes at particular wavelengths in the spectrum. Consequently, it is essential to provide a detection apparatus with the capability of filtering incident light to obtain a signal with a narrow bandwidth centered about one or more specific wavelengths and with a high rejection for the off-band portions of the received light. Another typical application where such a filtering device is needed is in optical communications systems, where a carrier optical beam may be multiplexed with a plurality of different signals, each modulating a different frequency within the beam spectrum. A narrow band filter is utilized to separate information encoded on a particular wavelength from the remainder of the carrier beam.
A number of optical systems of current interest require a filter with an extremely narrow bandwidth and a wide angular field. In the study of solar physics, for example, the distribution of hydrogen is measured by photographing the solar corona in the light of the H.sub..varies. (.lambda.=6563 .ANG.) spectral line. Since a large amount of light is present in neighboring wavelengths, a filter which passes a very narrow bandwidth (approximately 1 .ANG.) is required to attain reasonable discrimination. Another application for such filters is in those laser communications systems where the signal information must be transmitted through a random medium, such as the earth's atmosphere or sea water. The signal is carried by a scattered component of the laser radiation, which will appear to come from a wide field of view of up to several steradians. Optical communications under such severe circumstances demand a filter with not only an exceptionally narrow bandwidth, to reject unwanted background light and thereby increase the signal-to-noise ratio, but also a large angular aperture to facilitate the receipt of as much of the signal energy as is possible. Birefringent filters offer the potential to satisfy such demanding performance requirements.
The birefringent filters which are known in the prior art are composed of birefringent crystal plates (wave plates) placed between pairs of polarizers. The two basic versions of such filters which have generally been utilized are the Lyot-Ohman and Solc filters, which are designed to operate through the interference of polarized light. To achieve the necessary polarization interference, these filter designs must introduce a phase retardation between the respective components of the light polarized parallel to the fast and slow axes of the crystal when radiation passes through it. Since the phase retardation introduced by a wave plate is directly proportional to the birefringence of the crystal material from which the plate is fabricated, it has been thought desirable to utilize crystals with a large value of birefringence (i.e., with .vertline.n.sub.e -n.sub.o .vertline.large) in the construction of such filters. The materials which have most commonly been used in accordance with this approach are quartz, calcite, and ADP.
Unfortunately, however, filters designed with a maximum amount of birefringence (to achieve a narrow bandwidth) tend to exhibit a field of view which is too limited for many applications, due to the substantial change in the effective thickness of the wave plate which will occur in such a filter for light entering at angles of incidence away from the normal direction. This shortcoming has been ameliorated to some extent by adding wide field optical elements to such filters, but the narrow bandwidth (sub-Angstrom) birefringent filters known in the art have nevertheless provided only a moderate field of view and, in addition, have required inordinately thick crystal plates to achieve a sufficiently narrow bandwidth.
One possible solution to this problem is a unique type of new optical filter which is based on the accidental zero crossing of birefringence that occurs near the absorption band edge in certain uniaxial crystals. This filter was demonstrated in 1966 by C. H. Henry (see 143 Phys. Rev. 627 (1977)). The coupling of the ordinary and extraordinary electromagnetic waves in CdS at the crossing wavelength may be induced by an external magnetic field, an external electric field, or an applied stress. Since this coupling takes place only at the crossing wavelength, the effect can be used to produce a narrow bandpass filter when used with a pair of crossed polarizers. All such filter designs, however, require a perturbation such as stress, or a magnetic or electric field, to activate the effect. It would be desirable to provide a filter which exhibits a wide field of view and is capable of passing an extremely narrow bandwidth without the need for such an applied perturbation.